AC electrical distribution systems are widely used in residential and commercial settings. Typically, wall-mounted (or otherwise) AC electrical outlets, sockets, or receptacles are provided with which AC electrical appliances are connected. For example, a typical residential electrical power distribution system 20 of the prior art is schematically illustrated in FIG. 1. (As will be described, the remainder of the drawings illustrate the present invention.) The system 20 includes a meter 22 electrically connected to a source of electrical power (not shown). Connected to the meter is a circuit breaker panel 24 (or fuse box). The panel 24 includes a plurality of circuit breakers (or fuses, as the case may be). As is known in the art, electrical safety codes require that properly sized circuit breakers or fuses be used to protect the in-wall permanent wiring. Typically, a master breaker in the circuit breaker panel protects the feed wire and connections.
Pursuant to code requirements generally applicable in North America, the in-wall permanent wiring is minimum 14-gauge copper wire. Electrical devices such as appliances, electronics, lamps, extension cords, and strip outlets often use smaller diameter conductors in their internal wiring. For example, light-duty electrical extension cords incorporating 16-gauge wire are widely available. Typically, these smaller wires are not adequately protected by the branch circuit breakers or fuses. Overloads on smaller diameter wires in extension cords, lamps and other electrical devices can be the result of various causes, for example, improper product design, product life cycle fatigue, product abuse, excessive motor loads, physical damage to the wires, or high AC line voltage. In North America alone, overloads on smaller diameter wires cause thousands of fires every year.
For example, as can be seen in FIG. 1, the circuit breaker panel 24 has a plurality of outlets 26, 28, 30 electrically connected to the panel, and also, each of those outlets has one or more electrical devices (designated D1-D7 in FIG. 1) electrically connected therewith, via receptacles included in the outlets. By way of example, and as shown in FIG. 1, an electrical device D3 is electrically connected to a receptacle 32 in the outlet 26 via an electrical cord 34 and an extension cord 36. The cord 34 leads from electrical device D3 to a plug 38 connected to a receptacle 40 of the extension cord 36, i.e., the plug 38 is received in the receptacle 40. The other end of the extension cord 36 includes another plug which is received in a receptacle in the outlet 26. FIG. 1 also shows electrical devices D1 and D2 electrically connected to each of the receptacles in the wall outlet 28 via electrical cords 42, 44 respectively.
Extension cords are often improperly matched to the appliance load. Where the current flowing through an extension cord exceeds the extension cord's current rating, overheating of the internal conductors in the extension cord results and such overheating can cause burning of cord insulation and materials adjacent to the extension cord, resulting in fires.
FIG. 1 also shows a power strip 46 which is directly connected to a receptacle in the wall outlet 30. Each of devices D4-D7 is connected to power strip 46. As is known in the art, electrical devices D4-D7 can easily cause the circuit including the outlet 30 to overload, eventually resulting in a fire hazard.
For example, in the United States, a typical wall-mounted AC electrical outlet (and each receptacle included therein) is rated to handle 15 amperes of current. As described above, electrical protective devices such as circuit breakers (or fuses) are associated with the outlet, and will “trip” (i.e., disconnect) the outlet if a current overload through the outlet occurs. However, a light-duty extension may be connected to an outlet, and a number of electrical appliances may be connected. However, the extension cord may be rated for only 10 amperes of current. If three appliances are connected, with each appliance operating normally with a five-ampere current load, then all three appliances would draw 15 amperes of current through the extension cord when all three appliances are activated simultaneously. In these circumstances, because the current rating of the extension cord is exceeded, the conductors therein can overheat and cause a fire.
In this example, the circuit breaker does not trip because the current through the outlet has not exceeded the circuit breaker's 15A threshold for the outlet. In this example, therefore, it can be seen that even though the building electrical system, power strip, and each appliance may comply with applicable safety codes respectively, a fire can result from their use, because of how they are used together. In particular, it should be noted that the current overload protection provided by the circuit breaker does not help to avoid a fire in this example.
Other current overload faults can develop in other situations where the conventional overload protection provided by circuit breakers or fuses fails to prevent a fire. For instance, electrical appliances such as televisions, refrigerators, toasters, computers and the like can, and often do, develop internal faults that cause a “hot spot” within the appliance. An example is an appliance in which an electric motor drives rotating or moving parts, for example, in a clothes washing machine. With use over an extended time period, the bearings or bushings wear, and, eventually, lose lubrication. When this happens, the electric current needed to operate the motor increases in order to overcome the increased friction. As a result, the current load drawn by the appliance includes the normal operating current together with fault-induced current. This total current can exceed the current rating of the electrical cord of the appliance but still be insufficient to trip the relevant circuit breaker or fuse. Accordingly, this can result in a fire, because the cord overheats. Also, many appliances include combustible materials internally, which can ignite as a result of current overload.
In addition, improperly installed circuit breakers or fuses can allow unprotected overloads of in-wall wiring, electrical outlets, extension cords, or appliances. For example, if a 20-ampere circuit breaker or fuse is inadvertently installed on a standard branch circuit (i.e., wired with 14 gauge copper wire, typically rated for 15 amperes of current), then overloads can occur throughout the electrical system without proper protection, resulting in overheated wires and, possibly, fire.
In the prior art, residential electrical systems incorporate quick-disconnect power connectors for electrical devices to tap into the electrical distribution network. To minimize the risk of accidental access to high voltages, the wall outlets use female connectors with insulating cover plates. However, these connectors have line voltages which are easily accessible via the insertion of small conductive foreign objects, such as paper clips, hairpins, keys, cutlery or screwdrivers. In North America alone, thousands of persons require treatment for electrical shocks and many people die of electrocution due to contact with these line voltages.
Power distribution systems are subject to voltage variations due to device load switching and environmental changes such as lightning. Over-stressed grids can lead to brown-out and black-out conditions. Long distance electrical distribution can require a high line voltage, such as in rural areas. These various factors often result in power quality aberrations such as unexpected surges in line voltage, lower than acceptable line voltage, and higher than acceptable line voltage. Surge suppressors, such as metal oxide varistors (MOV), have been incorporated into many electrical devices to prevent damage to electronics and motors from sudden surges. However, longer duration, high energy surges can destroy the MOVs and remove the protection. Additionally, high and low line conditions can stress electrical devices and shorten their lives.
Therefore, there is a need for an electrical power distribution system which overcomes or mitigates at least one of the disadvantages of the prior art.